The present invention relates to an extrusion head for use in blow molding machines.
Conventional blow molding machines for blow molding thermoplastics into tubular forms or parisons may be divided in general into the following three types depending upon their accumulator type extrusion heads:
(I) the type in which the plastic material is fed into the extrusion head from the lower portion thereof, is accumulated below the plunger, and then is extruded;
(II) the type in which the plastic material flows through an annular passage, whose length or distance is varied depending upon the stroke of the plunger, below the plunger to be accumulated and then extruded; and
(III) the type in which the plunger has a double wall which defines an annular passage into which is accumulated the plastic material and from which is extruded the plastic material.
The extrusion head of the type (I) is disclosed in detail in for example in Japanese Patent Public Disclosure No. 49-107363, U.S. Pat. No. 3,386,132, as well as other disclosures. In FIG. 1, a prior art construction is shown having a cylindrical plunger c, which is annular in cross section, and is disposed between a head body a and a core b for vertical reciprocal movement, and the plastic material, which is plasticized or fluidized, is fed through a feed port d into the space defined below the plunger c, whereby the latter is caused to move upwards. Thereafter, the plunger c is forced to move downward whereby the plastic material is forced to flow through the nozzle f defined between the head body a and a mandrel e formed integral with the bore b, to thereby form a parison.
The extrusion head of the type (I) described above has certain disadvantages hereinafter described. That is, the first plastic material fed into the extrusion head through the feed port d remains in contact with the lower end surfaces of the plunger and the succeeding plastic material is accumulated below the previously fed plastic material. As a result, when the plunger c moves downward to extrude thus accumulated plastic material, the previously fed plastic material is extruded after the succeedingly fed plastic material. That is, the faster the plastic material is fed, the later it is extruded or the later the plastic material is fed, the faster it is extruded. As a result, the surface quality, dimensions, stability, strength etc. of the parison thus extruded are adversely affected.
In other words, in order to improve the desired requirements on parisons, such as surface quality, dimension, stability, strength etc., the plastic material must remain for a predetermined time interval within the extrusion head and the remaining time interval must be uniform from one shot to another. However, with the extrusion head of the type (I), the longer the plastic material remains in the extrusion head, the later it is extruded so that the plastic material fed into the extrusion head is not processed uniformly. As a result, the plastic material which has remained in the extrusion head for a relatively short time interval is extruded into a parison with enhanced surface roughness. On the other hand when the plastic material remains in the extrusion head for too a long time interval, its qualities are degraded. In addition, the seams or boundaries between the plastic material which has remained in the nozzle f and that accumulated above the nozzle f are developed when the plastic material is being extruded through the nozzle f so that the extruded parison has surfaces whose qualities are apparently different from each other. As a result, the end of finished product is not uniform in quality.
The extrusion head of the type (II) is disclosed in detail in for example in U.S. Pat. No. 3,611,494, and Japanese Utility Model Publication No. 52-26549. That is, as shown in prior art FIG. 2, the cylindrical plunger c, which is annular in cross section, is disposed for vertical movement between the head body a and the core b in such a way that an annular passage g may be defined between the plunger c and the core b. The length of distance of the annular passage g varies depending upon the stroke of the plunger c. The plastic material is fed into the extrusion head through the feed port d which is located at the upper portion of the head body a so that the plastic material may be accumulated successively in the annular passage g from the bottom protion thereof. As the plastic material is accumulated, the plunger c is raised upwards. When the plunger c is forced downward, the plastic material which has been fed into the extrusion head faster is extruded faster. That is, the faster the plastic material is fed, the faster it is extruded. As a result, the extrusion head of the type (II) is advantageous over the type (I) in that the plastic material may more uniformly remain for a predetermined time interval in the extrusion head.
However the extrusion head of the type (II) has also some disadvantages to be described hereinbelow. That is, since the annular passage g is defined between the core b and the plunger c, the plastic material tends to adhere the wall surfaces h of the annular passage g and it is almost impossible for the plunger c to scrape off the plastic material adhered to the wall surfaces h. As a result, when plastic materials of different compositions or colors are used, a cleaning agent or the like may be used in large quantity to remove the plastic material adhering to the wall surfaces h, with the resultant increase in molding cost.
The extrusion head of the type (III) is disclosed in detail in for instance U.S. Pat. No. 3,985,490. As shown in prior art FIG. 3, the plunger c is of the double-wall construction and has a longitudinal groove i which is in communication with the feed port d. The plastic material fed through the feed port d is introduced into the annular passage j defined between the walls of the plunger c and is accumulated successively from the bottom portion of the annular passage j, through the passage between the longitudinal groove i and the annular passage j.
The extrusion head, with the above construction, is advantageous in that the faster the plastic material is fed the faster it is extruded, but has a disadvantage to be described below. That is, as with the extrusion head of the type (II), the plunger c itself cannot scrape off the plastic materials adhering to the wall surfaces of the annular passage j so that a cleaning agent or the like must be used in a large quantity in order to remove the plastic materials adhering to the wall surfaces. In addition, the plastic material is introduced into the annular passage j through the longitudinal groove which in turn is communicated with the fed port d so that when the plunger c is not reciprocated over its full stroke, there remains the plastic material in the longitudinal groove i which will not flow at all. Since the plastic material remains for too a long time interval, its degradation or deterioration results. Furthermore, the plastic material which has flowed down through the longitudinal groove i can be permitted to flow into the annular passage j only when the plunger c is lifted to its upper stroke limit. As a result, the faster the plastic material is fed into the longitudinal groove i, the latter it is charged into the annular passage j.
The present invention overcomes the above described disadvantages as well as other problems encountered in the prior art extrusion heads in blow molding machines, and has for its object to provide an extrusion head for use in blow molding machines which may effectively scrape off or otherwise remove the plastic material adhering to the wall surfaces, thereby preventing the plastic material from remaining in the extrusion head over a predetermined time interval and reducing the quantity of a cleaning agent or the like required for removing the plastic material adhering to the wall surfaces when the plastic material of different composition or color is used and which may ensure a faster charged and faster extruded operation, thereby ensuring that all the plastic material fed may uniformly remain for a predetermined time interval. The present invention will become more apparent from the following description of some preferred embodiments thereof taken in conjunction with the accompanying drawings.